The present invention relates to transmitters used in industrial process control systems. In particular, the present invention relates to compensation for stray capacitance in transmitters which use a capacitance pressure differential sensor.
Two wire transmitters (as well as three wire and four wire transmitters) find widespread use in industrial process control systems. A two wire transmitter includes a pair of terminals which are connected in a current loop together with a power source and a load. The two wire transmitter is powered by the loop current flowing through the current loop. The two wire transmitter varies the magnitude of the loop current as a function of a parameter or condition which is sensed, for example sensed pressure.
Although a variety of operating ranges are possible, the most widely used two wire transmitter output varies from 4 to 20 milliamperes as a function of the sensed parameter. For example, typically 4 milliamperes represents a zero level and 20 milliamperes corresponds to a maximum output level.
Two wire transmitters have found widespread use in remote pressure sensing applications. A two wire transmitter uses a pressure differential sensor to sense pressure differential in an industrial environment. The two wire transmitter converts the sensed pressure differential into an electrical current level carried by the two wire current loop. Current flowing through the current loop can be sensed at a receiving unit and the pressure information conveyed to a system operator. One type of pressure differential sensor which is commonly used is a capacitive plate pressure sensor. U.S. Pat. No. 4,370,890, owned by the same assignee as the present invention, teaches one type of capacitance pressure differential sensor and is hereby incorporated by reference. The transfer function of the sensor is temperature dependent because the dielectric constant of oil in the sensor varies with temperature. Stray capacitance in the transmitter makes it difficult to account for this temperature dependence. It is known that in order to obtain accurate readings from a capacitance pressure differential sensor, stray capacitance must be canceled. Stray capacitance compensation is more difficult in two wire transmitters which use a removable module and a fixed module. In such a transmitter, the fixed module carries the sensor and the removable module carries transmitter circuitry. The capacitance values in a removable module may not match, and the effects of stray capacitance will need to be compensated after the removable modules are installed. This reduces the compatibility between different removable modules. There are two types of removable modules. One type (a "digital" model) uses digital circuitry to linearize the sensor output while the other type (an "analog" model) uses analog circuitry. The two types use two different methods to linearize the signal from the pressure differential sensor. This results in two different temperature coefficients for the transfer function of the sensors, which limits the compatibility between the two types of removable modules.
There is a continuing need for improved stray capacitance compensation which improves compatibility between different types of removable modules.